Method and apparatus for determining number of harq processes in wireless communication system

ABSTRACT

Provided are a method for determining the number of hybrid automatic repeat request (HARQ) processes in a carrier aggregated system configured with a plurality of serving cells, and an apparatus using such a method. The method receives data from a downlink subframe of a second serving cell, and transmits an ACK/NACK signal for the data from an uplink subframe of a first serving cell, wherein the first serving cell uses a first-type frame, the second serving cell uses a second-type frame, and the number of HARQ processes in the second serving cell are determined with respect to each subframe comprised in the second-type frame and on the basis of the number of downlink subframes comprised in each section comprising a set number of subframes.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for determining the number ofhybrid automatic repeat request (HARQ) processes in a wirelesscommunication system in which serving cells using a radio frame ofdifferent types are aggregated.

Related Art

Long term evolution (LTE) based on 3rd Generation Partnership Project(3GPP) Technical Specification (TS) is one of mobile communicationstandards. Meanwhile, 3GPP LTA-advanced (LTE-A) which is an evolution of3GPP LTE is currently underway. A technique introduced in 3GPP LTE-A isa carrier aggregation.

A carrier aggregation uses a plurality of component carriers. Acomponent carrier is defined by the center frequency and a bandwidth.One downlink component carrier or a pair of an uplink component carrierand a downlink component carrier correspond to one cell. It can be saidthat a terminal being served using a plurality of downlink componentcarriers is being served from a plurality of serving cells.

Meanwhile, an error compensation technique, which is to secure thereliability of wireless communications, includes a forward errorcorrection (FEC) scheme and automatic repeat request (ARQ) scheme. Inthe forward error correction (FEC) scheme, an error at a reception unitis corrected by adding extra error correction codes to information bits.The FEC scheme has an advantage in that there is less time delay and noinformation required to be transmitted and received between atransmission unit and a reception unit, however, it is a weak point thatsystem efficiency is low in good channel conditions. The ARQ scheme isstrong at transmission reliability, however, it causes time delay andthe system efficiency is low in poor channel conditions.

The hybrid automatic repeat request (HARQ) is a scheme in which FEC andARQ are coupled, it checks whether to include an error that datareceived by a physical layer cannot be decoded, and if the error isoccurred, efficiency can be increased by requesting retransmission.

A receiver in HARQ, if an error is not detected in data received,informs a success of the reception by transmitting an acknowledgementsignal to the reception acknowledgement. If an error is detected in datareceived, a receiver informs a transmitter of the error detected bytransmitting a NACK signal to the reception acknowledgement. If the NACKsignal is received, the transmitter may retransmit data.

Meanwhile, in the carrier aggregation in of the next generation wirelesscommunication systems, serving cells using TDD and serving cells usingFDD may be aggregated. That is, a plurality of serving cells that usesdifferent types of radio frames may be allocated to a UE. In aconventional art, it is defined to aggregate the serving cells that useidentical type of radio frames only in carrier aggregation. Accordingly,it is not considered of a method that determines the number of HARQprocesses in aggregation of serving cells that use different types ofradio frames.

In case that serving cells that use different types of radio frames areaggregated, it is problematic in which way the number of HARQ processes,which are simultaneously capable, is determined.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for determiningthe number of HARQ processes in wireless communication systems in whicha plurality of serving cells that uses different types of radio framesis aggregated.

In one aspect, provided is a method for determining a number of hybridautomatic repeat request (HARQ) processes in a carrier aggregationsystem in which a plurality of serving cells are configured. The methodincludes receiving data in a downlink subframe of a second serving celland transmitting an ACK/NACK signal in response to the data in an uplinksubframe of a first serving cell. A first type of frame is used in thefirst serving cell, and a second type of frame is used in the secondserving cell. The number of HARQ processes of the secondary cell isdetermined based on the number of downlink subframes that are includedin each of sections including a specific number of subframes based oneach subframe included in the second type of frame.

In another aspect, an apparatus is provided. The apparatus includes aradio frequency (RF) unit configured to transmit and receive a radiosignal and a processor connected to the RF unit. The processor isconfigured for receiving data in a downlink subframe of a second servingcell and transmitting an ACK/NACK signal in response to the data in anuplink subframe of a first serving cell. A first type of frame is usedin the first serving cell, and a second type of frame is used in thesecond serving cell. The number of HARQ processes of the secondary cellis determined based on the number of downlink subframes that areincluded in each of sections including a specific number of subframesbased on each subframe included in the second type of frame.

In wireless communication systems in which a plurality of serving cellsthat uses different types of radio frames is aggregated, the number ofHARQ processes of secondary cells can be effectively determined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of an FDD radio frame.

FIG. 2 shows the structure of a TDD radio frame.

FIG. 3 shows an example of a resource grid for one downlink slot.

FIG. 4 shows the structure of a DL subframe.

FIG. 5 shows the structure of an UL subframe.

FIG. 6 shows the channel structure of a PUCCH format 1b in a normal CP.

FIG. 7 shows the channel structure of PUCCH formats 2/2a/2b in a normalCP.

FIG. 8 illustrates the channel structure of a PUCCH format 3.

FIG. 9 shows an example of a comparison between a single carrier systemand a carrier aggregation system.

FIG. 10 shows one example in which a plurality of serving cells usesdifferent types of radio frames in a wireless communication system.

FIG. 11 shows a method of transmitting ACK/NACK for downlink datareceived through a primary cell.

FIG. 12 shows a method of transmitting ACK/NACK for downlink datareceived through a secondary cell.

FIG. 13 shows an example of ACK/NACK transmission timing when a primarycell is an FDD cell and a secondary cell is a TDD cell.

FIG. 14 shows an ACK/NACK transmission method based on Method 1.

FIG. 15 shows another example of ACK/NACK transmission timing when aprimary cell is an FDD cell and a secondary cell is a TDD cell.

FIG. 16 shows an ACK/NACK transmission method based on Method 2.

FIG. 17 illustrates a DL HARQ that is performed in a cell of 3GPP LTE.

FIG. 18 illustrates an example of determining the number of DL HARQprocesses in a FDD cell.

FIG. 19 illustrates an example of determining the number of DL HARQprocesses for each of UL-DL configurations 0 to 2 in a TDD cell.

FIG. 20 illustrates an example of determining the number of DL HARQprocesses for each of UL-DL configurations 3 to 5 in a TDD cell.

FIG. 21 illustrates an example of determining the number of DL HARQprocesses for UL-DL configuration 6 a TDD cell.

FIG. 22 illustrates an example of determining the number of DL HARQprocesses of the secondary cell.

FIG. 23 is a block diagram of a wireless apparatus in which theembodiments of the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

User Equipment (UE) can be fixed or can have mobility. UE can also becalled another term, such as a Mobile Station (MS), a Mobile Terminal(MT), a User Terminal (UT), a Subscriber Station (SS), a wirelessdevice, a Personal Digital Assistant (PDA), a wireless modem, or ahandheld device.

The BS commonly refers to a fixed station that communicates with UE. TheBS can also be called another tem, such as an evolved-NodeB (eNodeB), aBase Transceiver System (BTS), or an access point.

Communication from a BS to UE is called downlink (DL), and communicationfrom UE to a BS is called uplink (UL). A wireless communication systemincluding a BS and UE can be a Time Division Duplex (TDD) system or aFrequency Division Duplex (FDD) system. A TDD system is a wirelesscommunication system that performs UL and DL transmission/receptionusing different times in the same frequency band. An FDD system is awireless communication system that enables UL and DLtransmission/reception at the same time using different frequency bands.A wireless communication system can perform communication using radioframes.

FIG. 1 shows the structure of an FDD radio frame.

The FDD radio frame includes 10 subframes, and one subframe includes twoconsecutive slots. The slots within the radio frame are assigned indices0-19. The time that is taken for one subframe to be transmitted iscalled a Transmission Time Interval (TTI). A TTI can be a minimumscheduling unit. For example, the length of one subframe can be 1 ms,and the length of one slot can be 0.5 ms. Hereinafter, the FDD radioframe may be simply referred to as an FDD frame.

FIG. 2 shows the structure of a TDD radio frame.

Referring to FIG. 2, a downlink (DL) subframe and an uplink (UL)subframe coexist in a TDD radio frame used in TDD. Table 1 shows anexample of a UL-DL configuration of the radio frame.

TABLE 1 Uplink- Downlink- downlink to-uplink config- switch-pointSubframe n uration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D SU U U 1 5 ms D S U U D D S U U U 2 5 ms D S U D D D S U D D 3 10 ms  D SU U U D D D D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D D D DD 6 5 ms D S U U U D S U U D

In Table 1, ‘D’ indicates a DL subframe, ‘U’ indicates a UL subframe,and ‘S’ indicates a special subframe. When a UL-DL configuration isreceived from a BS, a UE can be aware of whether each subframe in aradio frame is a DL subframe or a UL subframe. Hereinafter, referencecan be made to Table 1 for a UL-DL configuration N (N is any one of 0 to6).

In the TDD frame, a subframe having an index #1 and an index #6 may be aspecial subframe, and includes a downlink pilot time slot (DwPTS), aguard period (GP), and an uplink pilot time slot (UpPTS). The DwPTS isused in initial cell search, synchronization, or channel estimation inUE. The UpPTS is used for channel estimation in a BS and for the uplinktransmission synchronization of UE. The GP is an interval in whichinterference occurring in UL due to the multi-path delay of a DL signalbetween UL and DL is removed. Hereinafter, the TDD radio frame may besimply referred to as a TDD frame.

FIG. 3 shows an example of a resource grid for one downlink slot.

Referring to FIG. 3, the downlink slot includes a plurality ofOrthogonal Frequency Division Multiplexing (OFDM) symbol in the timedomain and includes N_(RB) Resource Blocks (RBs) in the frequencydomain. The RBs includes one slot in the time domain and a plurality ofconsecutive subcarrier in the frequency domain in a resource allocationunit. The number of RBs N_(RB) included in the downlink slot depends ona downlink transmission bandwidth N^(DL) configured in a cell. Forexample, in an LTE system, the N_(RB) can be any one of 6 to 110. Anuplink slot can have the same structure as the downlink slot.

Each element on the resource grid is called a Resource Element (RE). TheRE on the resource grid can be identified by an index pair (k,l) withina slot. Here, k (k=0, N_(RB)×12-1) is a subcarrier index within thefrequency domain, and 1 (1=0, . . . , 6) is an OFDM symbol index withinthe time domain.

Although 7×12 REs including 7 OFDM symbols in the time domain and 12subcarrier in the frequency domain have been illustrated as beingincluded in one RB in FIG. 3, the number of OFDM symbols and the numberof subcarriers within an RB are not limited thereto. The number of OFDMsymbols and the number of subcarriers can be changed in various waysdepending on the length of a CP, frequency spacing, etc. In one OFDMsymbol, one of 128, 256, 512, 1024, 1536, and 2048 can be selected andused as the number of subcarriers.

FIG. 4 shows the structure of a DL subframe.

Referring to FIG. 4, a downlink (DL) subframe is divided into a controlregion and a data region in the time domain. The control region includesa maximum of former 3 (maximun 4 according to circumstances) 01-DMsymbols of a first slot within a subframe, but the number of OFDMsymbols included in the control region can be changed. A control channeldifferent from a physical downlink control channel (PDCCH) is allocatedto the control region, and a physical downlink shared channel (PDSCH) isallocated to the data region.

As disclosed in 3GPP TS 36.211 V8.7.0, in 3GPP LTE, physical channelscan be divided into a physical downlink shared channel (PDSCH) and aphysical uplink shared channel (PUSCH), that is, data channels, and aphysical downlink control channel (PDCCH), a physical control formatindicator channel (PCFICH), a physical hybrid-ARQ indicator channel(PHICH), and a physical uplink control channel (PUCCH), that is, controlchannels.

A PCFICH that is transmitted in the first OFDM symbol of a subframecarries a Control Format Indicator (CFI) regarding the number of OFDMsymbols (i.e., the size of a control region) that are used to sendcontrol channels within the subframe. UE first receives a CFI on aPCFICH and then monitors PDCCHs. Unlike in a PDCCH, a PCFICH is notsubject to blind decoding, but is transmitted through the fixed PCFICHresources of a subframe.

A PHICH carries a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal for an uplink HybridAutomatic Repeat reQuest (HARQ). An ACK/NACK signal for uplink (UL) dataon a PUSCH which is transmitted by UE is transmitted on a PHICH.

A physical broadcast channel (PBCH) is transmitted in the former 4 OFDMsymbols of a second slot within the first subframe of a radio frame. ThePBCH carries system information that is essential for UE to communicatewith a BS, and system information transmitted through a PBCH is called aMaster Information Block (MIB). In contrast, system informationtransmitted on a PDSCH indicated by a PDCCH is called a SystemInformation Block (SIB).

Control information transmitted through a PDCCH is called DownlinkControl Information (DCI). DCI can include the resource allocation of aPDSCH (this is also called a DL grant), the resource allocation of aPUSCH (this is also called an UL grant), a set of transmit power controlcommands for individual MSs within a specific UE group and/or theactivation of a Voice over Internet Protocol (VoIP).

FIG. 5 shows the structure of an UL subframe.

Referring to FIG. 5, the UL subframe can be divided into a controlregion to which a physical uplink control channel (PUSCH) for carryinguplink control information is allocated and a data region to which aphysical uplink shared channel (PUSCH) for carrying user data isallocated in the frequency domain.

A PUCCH is allocated with an RB pair in a subframe. RBs that belong toan RB pair occupy different subcarriers in a first slot and a secondslot. An RB pair has the same RB index m.

In accordance with 3GPP TS 36.211 V8.7.0, a PUCCH supports multipleformats. A PUCCH having a different number of bits in each subframe canbe used according to a modulation scheme that is dependent on a PUCCHformat.

Table 2 below shows an example of modulation schemes and the number ofbits per subframe according to PUCCH formats.

TABLE 2 Number of bits PUCCH format Modulation scheme per subframe 1 N/A N/A 1a BPSK 1 1b QPSK 2 2  QPSK 20 2a QPSK + BPSK 21 2b QPSK + QPSK22

The PUCCH format 1 is used to send a Scheduling Request (SR), the PUCCHformats 1a/1b are used to send an ACK/NACK signal for an HARQ, the PUCCHformat 2 is used to send a CQI, and the PUCCH formats 2a/2b are used tosend a CQI and an ACK/NACK signal at the same time. When only anACK/NACK signal is transmitted in a subframe, the PUCCH formats 1a/1bare used. When only an SR is transmitted, the PUCCH format 1 is used.When an SR and an ACK/NACK signal are transmitted at the same time, thePUCCH format 1 is used. In this case, the ACK/NACK signal is modulatedinto resources allocated to the SR and is then transmitted.

All the PUCCH formats use the Cyclic Shift (CS) of a sequence in eachOFDM symbol. A CS sequence is generated by cyclically shifting a basesequence by a specific CS amount. The specific CS amount is indicated bya CS index.

An example in which a base sequence r_(u)(n) has been defined is thesame as the following equation.

r _(u)(n)=e ^(jb(n)π/4)   [Equation 1]

Here, u is a root index, n is an element index wherein 0≦n≦N−1, and N isthe length of the base sequence. b(n) is defined in section 5.5 of 3GPPTS 36.211 V8.7.0.

The length of a sequence is the same as the number of elements includedin the sequence. U can be determined by a cell identifier (ID), a slotnumber within a radio frame, etc.

Assuming that a base sequence is mapped to one resource block in thefrequency domain, the length N of the base sequence becomes 12 becauseone resource block includes 12 subcarriers. A different base sequence isdefined depending on a different root index.

A CS sequence r(n, I_(cs)) can be generated by cyclically shifting thebase sequence r(n) as in Equation 2.

$\begin{matrix}{{{r\left( {n,I_{cs}} \right)} = {{r(n)} \cdot {\exp \left( \frac{j\; 2\pi \; I_{cs}n}{N} \right)}}},{0 \leq I_{cs} \leq {N - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Here, I_(cs) is a CS index indicative of a CS amount (0≦I_(cs)≦N−1).

An available CS index of a base sequence refers to a CS index that canbe derived from the base sequence according to a CS interval. Forexample, the length of a base sequence is 12 and a CS interval is 1, atotal number of available CS indices of the base sequence becomes 12.Or, if the length of a base sequence is 12 and a CS interval is 2, atotal number of available CS indices of the base sequence becomes 6.

FIG. 6 shows the channel structure of the PUCCH format 1b in a normalCP.

One slot includes 7 OFDM symbols, the 3 OFDM symbols become ReferenceSignal (RS) OFDM symbols for a reference signal, and the 4 OFDM symbolsbecome data OFDM symbols for an ACK/NACK signal.

In the PUCCH format 1b, a modulation symbol d(0) is generated byperforming Quadrature Phase Shift Keying (QPSK) modulation on an encoded2-bit ACK/NACK signal.

A CS index_(Is) can vary depending on a slot number ‘its’ within a radioframe and/or a symbol index ‘1’ within a slot.

In a normal CP, 4 data OFDM symbols for sending an ACK/NACK signal arepresent in one slot. It is assumed that corresponding CS indices inrespective data OFDM symbols are I_(cs0), I_(cs1), I_(cs2), and I_(cs3).

The modulation symbol d(0) is spread into a CS sequence r(n,Ics).Assuming that a 1-dimensional spread sequence corresponding to an(i+1)^(th) OFDM symbol is m(i) in a slot,

{m(0), m(1), m(2), m(3)}={d(0)r(n,I_(cs0)), d(0)r(n,I_(cs1)),d(0)r(n,I_(cs2)), d(0)r(n,I_(cs3))} can be obtained.

In order to increase a UE capacity, the 1-dimensional spread sequencecan be spread using an orthogonal sequence. The following sequence isused as an orthogonal sequence w_(i)(k) (i is a sequence index, 0≦k≦K−1)wherein a spreading factor K=4.

TABLE 3 Index (i) [w_(i)(0), w_(i)(1), w_(i)(2), w_(i)(3)] 0 [+1, +1,+1, +1] 1 [+1, −1, +1, −1] 2 [+1, −1, −1, +1]

The following sequence is used as an orthogonal sequence w_(i)(k) (i isa sequence index, 0≦k≦K−1) wherein a spreading factor K=3.

TABLE 4 Index (i) [w_(i)(0), w_(i)(1), w_(i)(2)] 0 [+1, +1, +1] 1 [+1,e^(j2π/3), e^(j4π/3)] 2 [+1, e^(j4π/3), e^(j2π/3)]

A different spreading factor can be used in each slot.

Accordingly, assuming that a specific orthogonal sequence index i isgiven, 2-dimensional spread sequences {s(0), s(1), s(2), s(3)} can beexpressed as follows.

{s(0),s(1),s(2),s(3)}={w _(i)(0)m(0),w _(i)(1)m(1),w _(i)(2)m(2),w_(i)(3)m(3)}

The 2-dimensional spread sequences {s(0), s(1), s(2), s(3)} are subjectto IFFT and then transmitted in a corresponding OFDM symbol.Accordingly, an ACK/NACK signal is transmitted on a PUCCH.

A reference signal having the PUCCH format 1b is also transmitted byspreading the reference signal into an orthogonal sequence aftercyclically shifting a base sequence r(n). Assuming that CS indicescorresponding to 3 RS OFDM symbols are I_(cs4), I_(cs5), and I_(cs6), 3CS sequences r(n,I_(cs4)), r(n,I_(cs5)), r(n,I_(cs6)) can be obtained.The 3 CS sequences are spread into an orthogonal sequence w^(RS) _(i)(k)wherein K=3.

An orthogonal sequence index i, a CS index I_(cs), and an RB index m areparameters necessary to configure a PUCCH and are also resources used toclassify PUCCHs (or MSs). If the number of available CSs is 12 and thenumber of available orthogonal sequence indices is 3, a PUCCH for atotal of 36 MSs can be multiplexed with one RB.

In 3GPP LTE, a resource index n⁽¹⁾ _(PUCCH) is defined so that UE canobtain the three parameters for configuring a PUCCH. The resource indexn⁽¹⁾ _(PUCCH)=n_(CCE)N⁽¹⁾ _(PUCCH), wherein n_(CCE) is the number of thefirst CCE used to send a corresponding PDCCH (i.e., PDCCH including theallocation of DL resources used to received downlink data correspondingto an ACK/NACK signal), and N⁽¹⁾ _(PUCCH) is a parameter that isinformed of UE by a BS through a higher layer message.

Time, frequency, and code resources used to send an ACK/NACK signal arecalled ACK/NACK resources or PUCCH resources. As described above, anindex of ACK/NACK resources (called an ACK/NACK resource index or PUCCHindex) used to send an ACK/NACK signal on a PUCCH can be represented asat least one of an orthogonal sequence index i, a CS index I_(cs), an RBindex m, and an index for calculating the 3 indices. ACK/NACK resourcescan include at least one of an orthogonal sequence, a CS, a resourceblock, and a combination of them.

FIG. 7 shows the channel structure of the PUCCH formats 2/2a/2b in anormal CP.

Referring to FIG. 7, in a normal CP, OFDM symbols 1 and 5 (i.e., secondand sixth OFDM symbols) are used to send a demodulation reference signal(DM RS), that is, an uplink reference signal, and the remaining OFDMsymbols are used to send a CQI. In the case of an extended CP, an OFDMsymbol 3 (fourth symbol) is used for a DM RS.

10 CQI information bits can be subject to channel coding at a ½ coderate, for example, thus becoming 20 coded bits. Reed-Muller code can beused in the channel coding. Next, the 20 coded bits are scramble andthen subject to QPSK constellation mapping, thereby generating a QPSKmodulation symbol (d(0) to d(4) in a slot 0). Each QPSK modulationsymbol is modulated in a cyclic shift of a base RS sequence ‘r(n)’having a length of 12, subject to IFFT, and then transmitted in each of10 SC-FDMA symbols within a subframe. Uniformly spaced 12 CSs enable 12different MSs to be orthogonally multiplexed in the same PUCCH RB. Abase RS sequence ‘r(n)’ having a length of 12 can be used as a DM RSsequence applied to OFDM symbols 1 and 5.

FIG. 8 shows an example of a channel structure of a PUCCH format 3.

Referring to FIG. 8, the PUCCH format 3 is a PUCCH format which uses ablock spreading scheme. The block spreading scheme means a method ofspreading a symbol sequence, which is obtained by modulating a multi-bitACK/NACK, in a time domain by using a block spreading code.

In the PUCCH format 3, a symbol sequence (e.g., ACK/NACK symbolsequence) is transmitted by being spread in the time domain by using theblock spreading code. An orthogonal cover code (OCC) may be used as theblock spreading code. Control signals of several UEs may be multiplexedby the block spreading code. In the PUCCH format 2, a symbol (e.g.,d(0), d(1), d(2), d(3), d(4), etc., of FIG. 7) transmitted in each datasymbol is different, and UE multiplexing is performed using the cyclicshift of a constant amplitude zero auto-correlation (CAZAC) sequence. Incontrast, in the PUCCH format 3, a symbol sequence including one or moresymbols is transmitted in a frequency domain of each data symbol, thesymbol sequence is spread in a time domain by using the block spreadingcode, and UE multiplexing is performed. An example in which 2 RS symbolsare used in one slot has been illustrated in FIG. 8, but the presentinvention is not limited thereto. 3 RS symbols may be used, and an OCChaving a spreading factor value of 4 may be used. An RS symbol may begenerated from a CAZAC sequence having a specific cyclic shift and maybe transmitted in such a manner that a plurality of RS symbols in thetime domain has been multiplied by a specific OCC.

Now, a carrier aggregation system is described. The carrier aggregationsystem is also called a multiple carrier system.

A 3GPP LTE system supports a case where a DL bandwidth and a ULbandwidth are differently configured, but one component carrier (CC) isa precondition in this case. A 3GPP LTE system supports a maximum of 20MHz and may be different in a UL bandwidth and a DL bandwidth, butsupports only one CC in each of UL and DL

A carrier aggregation (also called a bandwidth aggregation or a spectrumaggregation) supports a plurality of CCs. For example, if 5 CCs areallocated as the granularity of a carrier unit having a 20 MHzbandwidth, a maximum of a 100 MHz bandwidth may be supported.

FIG. 9 shows an example of a comparison between a single carrier systemand a carrier aggregation system.

A carrier aggregation system (FIG. 9 (b)) has been illustrated asincluding three DL CCs and three UL CCs, but the number of DL CCs and ULCCs is not limited. A PDCCH and a PDSCH may be independently transmittedin each DL CC, and a PUCCH and a PUSCH may be independently transmittedin each UL CC. Or, a PUCCH may be transmitted only through a specific ULCC.

Since three pairs of DL CCs and UL CCs are defined, it can be said thata UE is served from three serving cells. Hereinafter, a cell which isconfigured to provide a service to a user equipment is referred to aserving cell.

The UE may monitor PDCCHs in a plurality of DL CCs and receive DLtransport blocks through the plurality of DL CCs at the same time. TheUE may send a plurality of UL transport blocks through a plurality of ULCCs at the same time.

A pair of a DL CC #A and a UL CC #A may become a first serving cell, apair of a DL CC #B and a UL CC #B may become a second serving cell, anda DL CC #C and a UL CC#C may become a third serving cell. Each servingcell may be identified by a cell index (CI). The CI may be unique withina cell or may be UE-specific.

The serving cell may be divided into a primary cell and a secondarycell. The primary cell is a cell on which the UE performs an initialconnection establishment procedure or initiates a connectionre-establishment procedure, or a cell designated as a primary cell in ahandover process. The primary cell is also called a reference cell. Thesecondary cell may be configured after an RRC connection has beenestablished and may be used to provide additional radio resources. Atleast one primary cell is always configured, and a secondary cell may beadded/modified/released in response to higher layer signaling (e.g., anRRC message). The CI of the primary cell may be fixed. For example, thelowest CI may be designated as the CI of the primary cell.

The primary cell includes a downlink primary component carrier (DL PCC)and an uplink PCC (UL PCC) in view of a CC. The secondary cell includesonly a downlink secondary component carrier (DL SCC) or a pair of a DLSCC and a UL SCC in view of a CC. Hereinafter, the term, ‘cell’ may bemixed with the term ‘component carrier (CC)’.

As described above, the carrier aggregation system may support aplurality of CCs, that is, a plurality of serving cells unlike thesingle carrier system.

Such a carrier aggregation system may support cross-carrier scheduling.The cross-carrier scheduling is a scheduling method capable ofperforming resource allocation of a PDSCH transmitted through adifferent component carrier through a PDCCH transmitted through aspecific component carrier and/or resource allocation of a PUSCHtransmitted through other component carriers except for a componentcarrier fundamentally linked with the specific component carrier. Thatis, the PDCCH and the PDSCH may be transmitted through different DL CCs,and a PUSCH may be transmitted through a UL CC different from a UL CClinked with a DL CC to which a PDCCH including a UL is transmitted. Asdescribed above, in a system for supporting the cross-carrierscheduling, the PDCCH needs a carrier indicator indicating thatPDSCH/PUSCH are transmitted through a certain DL CC/UL CC. Hereinafter,a field including the carrier indicator refers to a carrier indicationfield (CIF).

The carrier aggregation system that supports the cross-carrierscheduling may include a carrier indication field (CIF) to theconventional downlink control information (DCI). In a system thatsupports the cross-carrier scheduling, for example, LTE-A system, 3 bitsmay be extended since the CIF is added to the conventional DCI format(i.e., the DCI format used in LTE), and the PDCCH structure may reusethe conventional coding method, resource allocation method (i.e.,resource mapping based on the CCE), and the like.

A BS may set a PDCCH monitoring DL CC (monitoring CC) group. The PDCCHmonitoring DL CC group is configured by a part of all aggregated DL CCs.If the cross-carrier scheduling is configured, the UE performs PDCCHmonitoring/decoding for only a DL CC included in the PDCCH monitoring DLCC group. That is, the BS transmits a PDCCH with respect to aPDSCH/PUSCH to be scheduled through only the DL CCs included in thePDCCH monitoring DL CC group. The PDCCH monitoring DL CC group may beconfigured in a UE-specific, UE group-specific, or cell-specific manner.

Non-cross carrier scheduling (NCSS) is a scheduling method capable ofperforming resource allocation of a PDSCH transmitted through a specificcomponent carrier through a PDCCH transmitted through the specificcomponent carrier and/or resource allocation of a PDSCH transmittedthrough a component carrier fundamentally linked with the specificcomponent carrier.

ACK/NACK transmission for HARQ in 3GPP LTE Time Division Duplex (TDD) isdescribed below.

In TDD, unlike in a Frequency Division Duplex (FDD), a DL subframe andan UL subframe coexist in one radio frame. In general, the number of ULsubframes is smaller than that of DL subframes. Accordingly, inpreparation for a case where UL subframes for sending an ACK/NACK signalare not sufficient, a plurality of ACK/NACK signals for DL transportblocks received in a plurality of DL subframes is transmitted in one ULsubframe.

In accordance with section 10.1 of 3GPP TS 36.213 V8.7.0 (2009-05), twoACK/NACK modes: ACK/NACK bundling and ACK/NACK multiplexing areinitiated.

In ACK/NACK bundling, UE sends ACK if it has successfully decoded allreceived PDSCHs (i.e., DL transport blocks) and sends NACK in othercases. To this end, ACK or NACKs for each PDSCH are compressed throughlogical AND operations.

ACK/NACK multiplexing is also called ACK/NACK channel selection (orsimply channel selection). In accordance with ACK/NACK multiplexing, UEselects one of a plurality of PUCCH resources and sends ACK/NACK.

Table below shows DL subframes n-k associated with an UL subframe naccording to an UL-DL configuration in 3GPP LTE, wherein kεK and M isthe number of elements of a set K.

TABLE 5 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 0 — — 6 — 4 —— 6 — 4 1 — — 7, 6 4 — — — 7, 6 4 — 2 — — 8, 7, 4, 6 — — — — 8, 7, 4, 6— — 3 — — 7, 6, 11 6, 5 5, 4 — — — — — 4 — — 12, 8, 7, 11 6, 5, 4, 7 — —— — — — 5 — — 13, 12, 9, 8, 7, 5, 4, — — — — — — — 11, 6 6 — — 7 7 5 — —7 7 —

It is assumed that M DL subframes are associated with the UL subframe nand, for example, M=3. In this case, UE can obtain 3 PUCCH resourcesn⁽¹⁾ _(PUCCH,0), n⁽¹⁾ _(PUCCH,1), and n⁽¹⁾ _(PUCCH,2) because it canreceive 3 PDCCHs from 3 DL subframes. In this case, an example ofACK/NACK channel selection is the same as the following table.

TABLE 6 HARQ-ACK(0), HARQ-ACK(1), HARQ-ACK(2) n⁽¹⁾ _(PUCCH) b(0), b(1)ACK, ACK, ACK n⁽¹⁾ _(PUCCH, 2) 1, 1 ACK, ACK, NACK/DTX n⁽¹⁾ _(PUCCH, 1)1, 1 ACK, NACK/DTX, ACK n⁽¹⁾ _(PUCCH, 0) 1, 1 ACK, NACK/DTX, NACK/DTXn⁽¹⁾ _(PUCCH, 0) 0, 1 NACK/DTX, ACK, ACK n⁽¹⁾ _(PUCCH, 2) 1, 0 NACK/DTX,ACK, NACK/DTX n⁽¹⁾ _(PUCCH, 1) 0, 0 NACK/DTX, NACK/DTX, ACK n⁽¹⁾_(PUCCH, 2) 0, 0 DTX, DTX, NACK n⁽¹⁾ _(PUCCH, 2) 0, 1 DTX, NACK,NACK/DTX n⁽¹⁾ _(PUCCH, 1) 1, 0 NACK, NACK/DTX, NACK/DTX n⁽¹⁾ _(PUCCH, 0)1, 0 DTX, DTX, DTX N/A N/A

In the above table, HARQ-ACK(i) indicates ACK/NACK for an i^(th) DLsubframe of M DL subframes. Discontinuous transmission (DTX) means thata DL transport block has not been received on a PDSCH in a correspondingDL subframe or that a corresponding PDCCH has not been detected. Inaccordance with Table 6, 3 PUCCH resources n⁽¹⁾ _(PUCCH,0), n⁽¹⁾_(PUCCH,1), and n⁽¹⁾ _(PUCCH,2) are present, and b(0), b(1) are two bitstransmitted using a selected PUCCH.

For example, when UE successfully receives all 3 DL transport blocks in3 DL subframes, the UE performs QPSK modulation on bits (1,1) using n⁽¹⁾_(PUCCH,2) and sends them on a PUCCH. If UE fails in decoding a DLtransport block in a first (i=0) DL subframe, but succeeds in decodingthe remaining transport blocks, the UE sends bits (1,0) on a PUCCH usingn⁽¹⁾ _(PUCCH,2). That is, in the existing PUCCH format 1b, only ACK/NACKof 2 bits can be transmitted. However, in channel selection, allocatedPUCCH resources are linked to an actual ACK/NACK signal in order toindicate more ACK/NACK states. This channel selection is also referredto as channel selection using the PUCCH format 1b.

In ACK/NACK channel selection, if at least one ACK is present, NACK andDTX are coupled. This is because all ACK/NACK states cannot berepresented by a combination of reserved PUCCH resources and a QPSKsymbol. If ACK is not present, however, DTX is decoupled from NACK.

The above-described ACK/NACK bundling and ACK/NACK multiplexing can beapplied in the case where one serving cell has been configured in UE inTDD.

For example, it is assumed that one serving cell has been configured(i.e., only a primary cell is configured) in UE in TDD, ACK/NACKbundling or ACK/NACK multiplexing is used, and M=1. That is, it isassumed that one DL subframe is associated with one UL subframe.

1) UE sends ACK/NACK in a subframe n if the UE detects a PDSCH indicatedby a corresponding PDCCH in a subframe n-k of a primary cell or detectsa Semi-Persistent Scheduling (SPS) release PDCCH. In LTE, a BS caninform UE that semi-persistent transmission and reception are performedin what subframes through a higher layer signal, such as Radio ResourceControl (RRC). Parameters given by the higher layer signal can be, forexample, the periodicity of a subframe and an offset value. When the UEreceives the activation or release signal of SPS transmission through aPDCCH after recognizing semi-persistent transmission through the RRCsignaling, the UE performs or releases SPS PDSCH reception or SPS PUSCHtransmission. That is, the UE does not immediately perform SPStransmission/reception although SPS scheduling is allocated theretothrough the RRC signaling, but when an activation or release signal isreceived through a PDCCH, performs SPS transmission/reception in asubframe that corresponds to frequency resources (resource block)according to the allocation of the resource block designated by thePDCCH, modulation according to MCS information, a subframe periodicityallocated through the RRC signaling according to a code rate, and anoffset value. Here, a PDCCH that releases SPS is called an SPS releasePDCCH, and a DL SPS release PDCCH that releases DL SPS transmissionrequires the transmission of an ACK/NACK signal.

Here, in the subframe n, UE sends ACK/NACK using the PUCCH formats 1a/1baccording to a PUCCH resource n^((1,p)) _(PUCCH). In n^((1,p)) _(PUCCH),p indicates an antenna port p. The k is determined by Table 5.

The PUCCH resource n^((1,p)) _(PUCCH) can be allocated as in thefollowing equation. P can be p0 or p1.

n ^((1,p=p0)) _(PUCCH)=(M−m−1)·N _(c) +m·N _(c+1) +n _(CCE) +N ⁽¹⁾_(PUCCH) for antenna port p=p0,

n ^((1,p=p1)) _(PUCCH)=(M−m−1)·N _(c) +m·N _(c+1) +n _(CCE)+1)+N ⁽¹⁾_(PUCCH) for antenna port p=p1,   [Equation 3]

In Equation 3, c is selected in such a way as to satisfyN_(c)≦n_(CCE)<N_(c+1) (antenna port p0) N_(c)≦(n_(CCE)+1)<N_(c+1)(antenna port p1) from among {0,1,2,3}. N⁽¹⁾ _(PUCCH) is a value set bya higher layer signal. N_(C)=max{0, floor [N^(DL) _(RB)·(N^(RB)_(sc)·c−4)/36]}. The N^(DL) _(RB) is a DL bandwidth, and N^(RB) _(sc) isthe size of an RB indicated by the number of subcarriers in thefrequency domain. n_(CCE) is a first CCE number used to send acorresponding PDCCH in a subframe n−km. m is a value that makes km thesmallest value in the set K of Table 5.

2) If UE detects an SPS PDSCH, that is, a PDSCH not including acorresponding PDCCH, in the DL subframe n−k of a primary cell, the UEcan send ACK/NACK in the subframe n using the PUCCH resource n^((1,p))_(PUCCH) as follows.

Since an SPS PDSCH does not include a scheduling PDCCH, UE sendsACK/NACK through the PUCCH formats 1a/1b according to n^((1,p)) _(PUCCH)that is configured by a higher layer signal. For example, 4 resources (afirst PUCCH resource, a second PUCCH resource, a third PUCCH resource,and a fourth PUCCH resource) can be reserved through an RRC signal, andone resource can be indicated through the Transmission Power Control(TPC) field of a PDCCH that activates SPS scheduling.

The following table is an example in which resources for channelselection are indicated by a TPC field value.

TABLE 7 TPC field value Resource for channel selection ‘00’ First PUCCHresource ‘01’ Second PUCCH resource ‘10’ Third PUCCH resource ‘11’Fourth PUCCH resource

For another example, it is assumed that in TDD, one serving cell isconfigured (i.e., only a primary cell is configured) in UE, ACK/NACKmultiplexing is used, and M>1. That is, it is assumed that a pluralityof DL subframes is associated with one UL subframe.

1) A PUCCH resource n⁽¹⁾ _(PUCCH) for sending ACK/NACK when UE receivesa PDSCH in a subframe n−k, (0≦i≦M−1) or detects a DL SPS release PDCCHcan be allocated as in the following equation. Here, k,εK, and the set Khas been described with reference to Table 5.

n ⁽¹⁾ _(PUCCH,i)=(M−i−1)·N _(c) +i·N _(c+1) +n _(CCE,i) +N ⁽¹⁾_(PUCCH)   [Equation 4]

Here, c is selected from {0,1,2,3} so that N_(c)≦n_(CCE,i)<N_(c+1) issatisfied. N⁽¹⁾ _(PUCCH) is a value set by a higher layer signal.N_(C)=max{0, floor [N^(DL) _(RB)·(N^(RB) _(sc)·c−4)/36]}. The N^(DL)_(RB) is a DL bandwidth, and N^(RB) _(sc) is the size of an RB indicatedby the number of subcarriers in the frequency domain. n_(CCE,i) is afirst CCE number used to send a corresponding PDCCH in the subframen−k_(i).

2) If UE receives a PDSCH (i.e., SPS PDSCH) not having a correspondingPDCCH in the subframe, n⁽¹⁾ _(PUCCH,i) is determined by a configurationgiven by a higher layer signal and Table 7.

If two or more serving cells have been configured in UE in TDD, the UEsends ACK/NACK using channel selection that uses the PUCCH format 1b orthe PUCCH format 3. Channel selection that uses the PUCCH format 1b usedin TDD can be performed as follows.

If a plurality of serving cells using channel selection that uses thePUCCH format 1b has been configured, when ACK/NACK bits are greater than4 bits, UE performs spatial ACK/NACK bundling on a plurality ofcodewords within one DL subframe and sends spatially bundled ACK/NACKbits for each serving cell through channel selection that uses the PUCCHformat 1b. Spatial ACK/NACK bundling means the compression of ACK/NACKfor each codeword through logical AND operations within the same DLsubframe.

If ACK/NACK bits are 4 bits or lower, spatial ACK/NACK bundling is notused and the ACK/NACK bits are transmitted through channel selectionthat uses the PUCCH format 1b.

If 2 or more serving cells using the PUCCH format 3 have been configuredin UE, when ACK/NACK bits are greater than 20 bits, spatial ACK/NACKbundling can be performed in each serving cell and ACK/NACK bitssubjected to spatial ACK/NACK bundling can be transmitted through thePUCCH format 3. If ACK/NACK bits are 20 bits or lower, spatial ACK/NACKbundling is not used and the ACK/NACK bits are transmitted through thePUCCH format 3.

<Channel Selection Using the PUCCH Format 1b Used in FDD>

If two serving cells using FDD have been configured in UE, ACK/NACK canbe transmitted through channel selection that uses the PUCCH format 1b.The UE can feed ACK/NACK for a maximum of 2 transport blocks, receivedin one serving cell, back to a BS by sending 2-bit (b(0)b(1))information in one PUCCH resource selected from a plurality of PUCCHresources. One codeword can be transmitted in one transport block. APUCCH resource can be indicated by a resource index n⁽¹⁾ _(PUCCH,i).Here, A is any one of {2, 3, 4}, and i is 0≦i≦(A−1). The 2-bitinformation is indicated as b(0)b(1).

HARQ-ACK(j) indicates an HARQ ACK/NACK response that is related to atransport block or DL SPS release PDCCH transmitted by a serving cell.The HARQ-ACK(j), the serving cell, and the transport block can have thefollowing mapping relationship.

TABLE 8 HARQ-ACK(j) A HARQ-ACK(0) HARQ-ACK(1) HARQ-ACK(2) HARQ-ACK(3) 2Transport block 1 Transport block 2 NA NA of primary cell of secondarycell 3 Transport block 1 Transport block 2 Transport block 3 NA ofserving cell 1 of serving cell 1 of serving cell 2 4 Transport block 1Transport block 2 Transport block 3 Transport block 4 of primary cell ofprimary cell of secondary cell of secondary cell

In Table 8, for example, in the case of A=4, HARQ-ACK(0) and HARQ-ACK(1)indicate ACK/NACKs for 2 transport blocks transmitted in a primary cell,and HARQ-ACK(2) and HARQ-ACK(3) indicate ACK/NACKs for 2 transportblocks transmitted in a secondary cell.

When UE receives a PDSCH or detects a DL SPS release PDCCH by detectinga PDCCH in a subframe ‘n−4’ of a primary cell, the UE sends ACK/NACKusing a PUCCH resource n⁽¹⁾ _(PUCCH,i). Here, n⁽¹⁾ _(PUCCH,i) isdetermined to be n_(CCE,i)+N⁽¹⁾ _(PUCCH). Here, n_(CCE,i) means an indexof the first CCE that is used to send a PDCCH by a BS, and N⁽¹⁾ _(PUCCH)is a value set through a higher layer signal. If a transmission mode ofa primary cell supports up to two transport blocks, a PUCCH resourcen⁽¹⁾ _(PUCCH,i+1) is given. Here, n⁽¹⁾ _(PUCCH,i+1) can be determined tobe n_(CCE,i)+1+N⁽¹⁾ _(PUCCH). That is, if a primary cell is set in atransmission mode in which a maximum of up to 2 transport blocks can betransmitted, 2 PUCCH resources can be determined.

If a PDCCH detected in a subframe ‘n−4’ of a primary cell is notpresent, a PUCCH resources n⁽¹⁾ _(PUCCH,i) for sending ACK/NACK for aPDSCH is determined by a higher layer configuration. If up to 2transport blocks are supported, a PUCCH resource n⁽¹⁾ _(PUCCH,i+1) canbe given as n⁽¹⁾ _(PUCCHi+1)=n⁽¹⁾ _(PUCCH,i+1).

If a PDSCH is received in a secondary cell by detecting a PDCCH in asubframe ‘n−4’, PUCCH resources n⁽¹⁾ _(PUCCH,i) and n⁽¹⁾ _(PUCCH,i+1)for a transmission mode in which up to 2 transport blocks are supportedcan be determined by a higher layer configuration.

Meanwhile, in the prior art, it was a precondition that a plurality ofserving cells configured in a UE uses radio frames having the same type.For example, it was a precondition that all of a plurality of servingcells configured in the UE use FDD frames or use TDD frames. In thenext-generation wireless communication system, however, different typesof radio frames may be used respectively in serving cells.

FIG. 10 shows one example in which a plurality of serving cells usedifferent types of radio frames in a wireless communication system.

Referring to FIG. 10, a primary cell PCell and a plurality of secondarycells SCell #1, . . . , SCell #N may be configured in a UE. In thiscase, the primary cell may operate in FDD and use an FDD frame, and thesecondary cells may operate in TDD and use TDD frames. The same UL-DLconfiguration may be used in the plurality of secondary cells. A DLsubframe (indicated by D) and a UL subframe (indicated by U) are presentin a 1:1 manner in the primary cell, but a DL subframe and a UL subframemay be present in a different ratio other than 1:1 in the secondarycells.

Table 9 below shows that ACK/NACK is transmitted in what a subframeaccording to a UL-DL configuration when one serving cell operates inTDD. Table 9 is equivalent to Table 5.

TABLE 9 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 0 4 6 — 4 6 —1 7 6 4 7 6 4 2 7 6 4 8 7 6 4 8 3 4 11  7 6 6 5 5 4 12  11  8 7 7 6 5 45 12  11  9 8 7 6 5 4 13  6 7 7 7 7 5

In Table 9, when a UE receives a PDSCH or a PDCCH (e.g., DL SPS releasePDCCH) necessary for an ACK/NACK response in a subframe n, the UE sendsACK/NACK in a subframe n+k(n). Each of the values of Table 9 indicatesthe k(n) value. For example, Table 9 indicates that if a UL-DLconfiguration is 0 and a PDSCH is received in a subframe 0, ACK/NACK istransmitted after four subframes elapse, i.e., in a subframe 4. Aspecific time is necessary in order for the UE to send ACK/NACK afterreceiving a PDSCH or a DL SPS release PDCCH. A minimum value of thisspecific time is hereinafter indicated as k_(min), and a value ofk_(min) may be four subframes. Four subframes, which is the minimumvalue of the specific time, are determined by considering a propagationdelay between the transmission terminal and the reception terminal, aprocessing time which is required for decoding at the receptionterminal. When looking at the timing of transmitting ACK/NACK in Table 9above, the ACK/NACK is mainly transmitted in the first uplink subframeafter a lapse of k_(min). However, the underlined figures do notindicate the first uplink subframe after a lapse of k_(min), butindicate the uplink subframe which is located at the next position. Thereason for this is to prevent from transmitting the ACK/NACK for toomany downlink subframes in one uplink subframe.

Meanwhile, since a UL subframe: DL subframe ratio is always 1:1 in FDD,ACK/NACK timing is determined as shown in the following table.

TABLE 9-1 Frame Subframe n Structure 0 1 2 3 4 5 6 7 8 9 FDD 4 4 4 4 4 44 4 4 4

That is, as shown in the above Table, k(n)=k_(min)=4 for all subframes.

Meanwhile, in the prior art, it was a precondition that all servingcells use radio frames having the same type, and ACK/NACK transmissiontiming, that is, HARQ timing, was determined based on this assumption.However, if a plurality of serving cells use different types of radioframes, it is necessary to determine which method will be used totransmit ACK/NACK.

It is hereinafter assumed that a primary cell and at least one secondarycell are configured in a UE in a wireless communication system. It isalso assumed that the primary cell uses an FDD frame and the secondarycell uses a TDD frame. Any one of the UL-DL configurations of Table 1may be used in the TDD frame. Hereinafter, only a relationship between aprimary cell and one secondary cell is illustrated, for convenience ofdescription, but this relationship may be applied to a relationshipbetween a primary cell and each of a plurality of secondary cells whenthe plurality of secondary cells are configured in the UE.

Under this assumption, first, a method of transmitting ACK/NACK fordownlink data received through a primary cell is described below.Hereinafter, the downlink data generally indicates a PDSCH that requestsan ACK/NACK response, a codeword included in a PDSCH, a DL SPS releasePDCCH indicating a DL SPS release and the like.

FIG. 11 shows a method of transmitting ACK/NACK for downlink datareceived through a primary cell.

Referring to FIG. 11, a BS sends downlink data in a subframe n of aprimary cell (S110). From a view of a UE, the downlink data is receivedin a subframe n of a DL PCC of the primary cell.

The UE decodes the downlink data and generates ACK/NACK for the downlinkdata (S120).

The UE sends the ACK/NACK in a subframe n+k_(PCC)(n) of the primary cell(S130).

The subframe n+k_(PCC)(n) of the primary cell is a subframe after aminimum delay time (this is called k_(min)) necessary for an ACK/NACKresponse has elapsed from a point of time at which the downlink data wasreceived. Here, the minimum delay time k_(min) may be four subframes.Accordingly, the UE may send the ACK/NACK in a subframe n+4 of a UL PCCof the primary cell.

That is, in the primary cell, as in the case where an HARQ is performedin conventional FDD, the ACK/NACK is transmitted in a subframe afterfour subframes elapse from a subframe in which data was received.

Now, a method of sending ACK/NACK when a UE receives downlink data in asecondary cell is described.

FIG. 12 shows a method of transmitting ACK/NACK for downlink datareceived through a secondary cell.

Referring to FIG. 12, a BS sends information about a UL-DL configurationof the secondary cell (S210). The secondary cell may need the UL-DLconfiguration information because it operates in TDD. The UL-DLconfiguration information may be transmitted through a higher layersignal, such as an RRC message.

A BS sends downlink data in a subframe n of the secondary cell (S220).

The UE decodes the downlink data and generates ACK/NACK for the downlinkdata (S230).

The UE may send the ACK/NACK to the BS through a subframe n+k_(SCC)(n)of a primary cell (S240). The subframe n+k_(SCC)(n) may be determined bythe following method.

<HARQ ACK/NACK Transmission Timing in System in which CCs UsingDifferent Frame Structures are Aggregated>

<Method 1>

Method 1 is a method in which a subframe n+k_(SCC)(n) complies withACK/NACK transmission timing in a primary cell. That is, Method 1 is amethod of configuring a UL subframe of the primary cell equal ton+k_(min) as the subframe n+k_(SCC)(n). In other words, if data isreceived in a subframe n of a secondary cell, ACK/NACK for the data istransmitted in the subframe n+k_(min) of the primary cell. Here, k_(min)may be, for example, four subframes.

FIG. 13 shows an example of ACK/NACK transmission timing when a primarycell is an FDD cell and a secondary cell is a TDD cell.

Referring to FIG. 13, it is assumed that a UL subframe of PCC in whichACK/NACK is transmitted for a DL data channel or DL control channelreceived in a DL subframe n of PCC is a subframe n+k_(PCC)(n). In caseof FDD, to avoid an ACK/NACK transmission delay, it may be set tok_(PCC)(n)=k_(min)=4 similarly in the conventional way.

It is assumed that a UL subframe of PCC in which ACK/NACK is transmittedfor a DL data channel or DL control channel received in a DL subframe nof SCC is a subframe n+k_(SCC)(n). Then, k_(SCC)(n) may comply withACK/NACK timing of FDD configured in PCC. That is, it may be set tok_(SCC)(n)=k_(min)=4. For example, ACK/NACK for a DL data channel or DLcontrol channel received in a subframe n 131 of SCC is transmitted in asubframe n+4 132 of PCC.

FIG. 14 shows an ACK/NACK transmission method based on Method 1.

Referring to FIG. 14, in a situation where a 1^(st) cell and a 2^(nd)cell are aggregated, data requiring ACK/NACK is received in a DLsubframe of the 2^(nd) cell (S161). Herein, the data requiring ACK/NACKcollectively refers to data requiring an ACK/NACK response such as aPDSCH, a transport block, and a DL SPS release PDCCH. The 1^(st) cell isan FDD cell using an FDD frame, and may be a primary cell. The 2^(nd)cell is a TDD cell using a TDD frame, and may be a secondary cell.

A UE transmits ACK/NACK for the data in a UL subframe of the 1^(st) celldetermined according to ACK/NACK timing of the 1^(st) cell (S162).

In accordance with Method 1, there is an advantage in that ACK/NACKdelay is minimized because ACK/NACK for downlink data received in thesecondary cell is always transmitted after k_(min) subframes elapse onthe basis of a point of time at which the downlink data was received.

Furthermore, in conventional TDD, if the number of DL subframesassociated with one UL subframe is many, there is a problem in that thenumber of ACK/NACKs that must be transmitted in the one UL subframe isincreased. However, Method 1 is advantageous in that ACK/NACKtransmission is distributed.

If the UL subframe of a primary cell in which ACK/NACK is transmitted isa subframe n, the number of ACK/NACK resources that need to be securedin the subframe n may be determined by a transmission mode of theprimary cell for a subframe n−k_(min) and a transmission mode in a DLsubframe of the secondary cell.

In accordance with Method 1, ACK/NACK timing applied to the UE may berepresented by changing Table 5 into Table 10 below.

TABLE 10 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 0 4 — — — 44 — — — 4 1 4 — — 4 4 4 — — 4 4 2 4 — 4 4 4 4 — 4 4 4 3 4 4 4 4 4 4 — —— 4 4 4 4 4 4 4 4 — — 4 4 5 4 4 4 4 4 4 — 4 4 4 6 4 — 4 4 4 — 4

That is, if a UL-DL configuration of the secondary cell is the same asany one of Table 10 and the primary cell uses an FDD frame, a subframe nis a subframe in which ACK/NACK is transmitted and a number indicated inthe subframe n indicates k_(min). Herein, the subframe n−k_(min)indicates a subframe in which downlink data, that is, the subject ofACK/NACK, is received. For example, in Table 10, a UL-DL configurationis 0, and 4 is written in a subframe 9. In this case, it indicates thatACK/NACK for downlink data received in the subframe 5 (=9-4) of thesecondary cell is transmitted in the subframe 9.

In accordance with Method 1, ACK/NACK timing applied to the UE may berepresented by changing Table 9 into Table 11 below.

TABLE 11 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 0 4 4 — 4 4— 1 4 4 4 4 4 4 2 4 4 4 4 4 4 4 4 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 44 4 4 4 4 4 4 4 6 4 4 4 4 4

In Table 11, a subframe n indicates a subframe in which downlink data isreceived. A subframe n+k_(SCC)(n) is a subframe in which ACK/NACK forthe downlink data is transmitted. Each of values in Table 11 indicates ak_(SCC)(n) value for the subframe n. For example, it indicates that, ifa UL-DL configuration is 0 and downlink data is received in the subframe1 of a secondary cell, ACK/NACK is transmitted in a subframe 5 (of aprimary cell) after four subframes elapse.

Tables 10 and 11 and FIG. 13 have a precondition that the radio frameboundaries of a secondary cell and a primary cell are the same. That is,it is a precondition that the radio frame of the primary cell issynchronized with the radio frame of the secondary cell. If the radioframe of the primary cell is not synchronized with the radio frame ofthe secondary cell, additional subframe delay (indicated by k_(add)) forcompensating for this asynchronization may be taken into consideration.That is, in Method 1, k_(SCC)(n) may be changed into k_(min)+k_(add).

Or, assuming that downlink data is received in the subframe n of asecondary cell and a subframe in which ACK/NACK for the downlink data istransmitted is n+k_(SCC)(n), if the k_(SCC)(n) is smaller thank_(min)+k_(add), scheduling may be limited so that the downlink data isnot transmitted in the subframe n of the secondary cell.

<Method 2>

Method 2 is a method of determining a subframe n+k_(SCC)(n) in whichACK/NACK is transmitted based on TDD ACK/NACK transmission timing in asecondary cell. That is, k_(SCC)(n) is determined as in Table 9, butactual ACK/NACK is transmitted through the UL PCC of a primary cell. Inother words, ACK/NACK for a DL data channel or DL control channelreceived in SCC may be transmitted in a UL subframe of PCC according toACK/NACK timing configured in SCC.

FIG. 15 shows another example of ACK/NACK transmission timing when aprimary cell is an FDD cell and a secondary cell is a TDD cell.

Referring to FIG. 15, it is assumed that a UL subframe of PCC in whichACK/NACK is transmitted for a DL data channel or DL control channelreceived in a DL subframe n of PCC is a subframe n+k_(PCC)(n). In caseof FDD, to avoid an ACK/NACK transmission delay, it may be set tok_(PCC)(n)=k_(min)=4 similarly in the conventional way.

In this case, ACK/NACK for a DL data channel or DL control channelreceived in a DL subframe n 141 of SCC may be transmitted in a ULsubframe n+k(n) 142 of SCC when ACK/NACK timing configured in SCC isapplied. In this case, the ACK/NACK is transmitted in a UL subframe 143of PCC at a time equal to that of the UL subframe n+k(n) 142.

FIG. 16 shows an ACK/NACK transmission method based on Method 2.

Referring to FIG. 16, in a situation where a 1^(st) cell and a 2^(nd)cell are aggregated, data requiring ACK/NACK is received in a DLsubframe of the 2^(nd) cell (S151). Herein, the data requiring ACK/NACKrefers to data requiring an ACK/NACK response such as a PDSCH, atransport block, and a DL SPS release PDCCH. The 1^(st) cell is an FDDcell using an FDD frame, and may be a primary cell. The 2^(nd) cell is aTDD cell using a TDD frame, and may be a secondary cell.

A UE transmits ACK/NACK for the data in a UL subframe of the 1^(st) celldetermined according to ACK/NACK timing applied when only the 2^(nd)cell is configured (S152).

Such a method has an advantage in that ACK/NACK timing for TDD CC may beequally applied irrespective of whether the TDD CC is used as a primarycell or secondary cell.

The number of resources for ACK/NACK transmission, which must be ensuredin a UL subframe of PCC, is determined according to whether a DLsubframe is present in a PCC/SCC at a subframe n and according to atransmission mode at the present DL subframe.

If the radio frame of a primary cell is not synchronized with that of asecondary cell, additional subframe delay (indicated by k_(add)) forcompensating for this asynchronization may be taken into consideration.The k_(add) may be a fixed value or may be a value set through an RRCmessage. In Method 2, assuming that k′_(SCC)(n)=k_(SCC)(n)+k_(add),ACK/NACK for downlink data received in the subframe n of the secondarycell may be represented as being transmitting in the UL subframen+k′_(SCC)(n) of the primary cell.

Or, assuming that downlink data is received in the subframe n of thesecondary cell and a subframe in which ACK/NACK for the downlink data istransmitted is n+k_(SCC)(n), if the k_(SCC)(n) is smaller thank_(min)+k_(add), scheduling may be limited so that the downlink data isnot transmitted in the subframe n of the secondary cell.

If Method 1 is used as the method of transmitting ACK/NACK in a primarycell and the method of transmitting ACK/NACK for a secondary cell, theACK/NACK for the primary cell and the secondary cell may comply with anACK/NACK transmission scheme used in FDD. For example, channel selectionmay be used in which the PUCCH format 1b used in FDD is used when aplurality of serving cells are configured in a UE. That is, ACK/NACK forthe secondary cell is transmitted using channel selection that uses thePUCCH format 1b through the primary cell without using a compressionscheme, such as ACK/NACK bundling. A compression scheme, such asACK/NACK bundling, may not be used because only one DL subframe isassociated with one UL subframe of the primary cell.

In contrast, if Method 2 is used as the method of transmitting ACK/NACKin a primary cell and the method of transmitting ACK/NACK for asecondary cell, the ACK/NACK for the primary cell and the secondary cellmay comply with an ACK/NACK transmission scheme used in TDD. Forexample, ACK/NACK may be transmitted through channel selection that usesthe PUCCH format 1b used when a plurality of serving cells areconfigured in TDD.

Whether to apply the aforementioned Methods 1 and 2 may be determinedaccording to whether to use cross carrier scheduling or non-crosscarrier scheduling. For example, Method 1 may be used in the crosscarrier scheduling and Method 2 may be used in the non-cross carrierscheduling.

If CCs to be aggregated use different frame structures (an aggregationof an FDD CC and a TDD CC), one CC may perform UL transmission andanother CC may perform DL reception in the same time duration (orsubframe). In this case, the UL transmission may have an effect on theDL reception. Therefore, it is not desirable to perform the ULtransmission and the DL reception simultaneously in contiguous frequencybands.

To solve this problem, preferably, frequency bands separated enough notto be interfered from each other are grouped, so that the same UL-DLconfiguration is used in one group and different UL-DL configurationsare used in different groups.

For example, if CCs #1 to #5 are aggregated in an ascending order of anallocated frequency band, the CCs #1 and #2 are grouped as a first groupand the CCs #3 to #5 are grouped as a second group, and all CCs in thefirst group use a UL-DL configuration 0, and all CCs in the second groupuse a UL-DL configuration 3. In this case, the CC #2 and the CC #3 maybe CCs separated enough not to be interfered from each other. In theabove example, a UE may have an independent RF module for each group,and may use a separate power amplifier. The UE may transmit one PUCCHfor each group, and in this case, a problem of a peak to average ratio(PAPR) increase does not occur even if a plurality of PUCCHs aretransmitted in uplink.

If the PUCCH is transmitted only with PCC, Method 1 may be applied, andif the PUCCH is transmitted in a specific UL CC of a group (of anon-contiguous frequency band) to which the PCC does not belong,ACK/NACK timing transmitted through the PUCCH may comply with ACK/NACKtiming corresponding to a DL subframe of the specific UL CC in which thePUCCH is transmitted.

<A Method for Determining the Number of DL HARQ Processes in Aggregationof Serving Cells that Use Different Frame Structures with Each Other>

FIG. 17 illustrates a DL HARQ that is performed in a cell of 3GPP LTE.

Referring to FIG. 17, a base station transmits a DL transmission blockin subframe n onto a PDSCH 412 that is indicated by a DL resourceallocation on a PDCCH 411 to a UE.

The UE transmits an ACK/NACK signal onto a PUCCH 420 in subframe n+4. Asan example, the resource of the PUCCH 420 that is used for thetransmission of ACK/NACK signal may be determined based on the resourceof the PDCCH 411 (for example, the first CCE index that is used for thetransmission of the PDCCH 411).

Even though the base station receives a NACK signal from the UE, it doesnot necessarily retransmit it in subframe n+8 unlike the UL HARQ. Here,an example is illustrated that a retransmission block is transmittedonto a PDSCH 432 that is indicated by the DL resource allocation on aPDCCH 431 in subframe n+9.

The UE transmits an ACK/NACK signal onto a PUCCH 440 in subframe n+13.

The following table shows a location of the shortest DL subframe aftersubframe 4 in each UL subframe in a TDD frame according to each of theUL-DL configurations. When a UE transmits an ACK/NACK in the ULsubframe, a base station is required to perform scheduling based onthis, which needs a certain amount of time, and the minimum value ofthis time is four subframes.

TABLE 12 Subframe n UL-DL Configuration 0 1 2 3 4 5 6 7 8 9 0 4 7 6 4 76 1 4 6 4 6 2 4 4 3 4 4 4 4 4 4 5 4 6 4 6 5 4 7 FDD 4 4 4 4 4 4 4 4 4 4

For example, according to UL-DL configuration 0, subframes 2, 3, 4, 7, 8and 9 are UL subframes, and the rest subframes are DL subframes (orspecial subframes). Based on UL-DL configuration 0 of the above table,subframe 4 indicates 6, and 6 represents that a shortest DL subframelocated after four subframes is a 6^(th) subframe after subframe 4(i.e., subframe 0 of the next frame) based on subframe 4. Likewise, inUL-DL configuration 6, subframe 8 indicates 7, and this represents thata shortest DL subframe located after four subframes is a 7^(th) subframeafter subframe 8 (i.e., subframe 5 of the next frame) based on subframe8.

That is, Table 12 above represents the shortest time to be taken by thebase station to perform a transmission according to the ACK/NACKresponse (i.e., transmitting new data, or retransmitting the existingdata) as a subframe unit, after the UE transmits the ACK/NACK in the DLHARQ.

Hereinafter, for the convenience of descriptions, the time of a DL HARQprocess to be taken is defined as follows. First, what a base stationtransmits data refers to an initial transmission. The above datamentioned refers to all things that are required for an ACK/NACKresponse. For example, it does not limit to data such as a transmissionblock, codeword, PDSCH, etc., but includes a control channel that isrequired for the ACK/NACK response like a DL SPS release PDCCH.

The UE transmits an ACK/NACK for the above data. The base station maytransmit new data or retransmit the data according to the ACK/NACK afterreceiving the ACK/NACK transmitted. The base station may transmit newdata, however, let's refer to a retransmission to transmit dataaccording to the ACK/NACK response for convenience sake.

Then, the time required for performing an identical DL HARQ process maybe defined by the time difference between the initial transmission andthe retransmission. The time required for performing the DL HARQ processmay refer to the sum of the time to be taken till the initialtransmission (it is marked to as k. Here, k is a unit of a subframe),which is an object of the ACK/NACK based on the UL subframe to which theACK/NACK is transmitted, and the time to be taken till retransmission(it is marked to as j. Here, j is a unit of a subframe) after theACK/NACK.

In the FDD, the proportion of DL subframe and UL subframe is 1:1 and theDL subframe and the UL subframes are sequentially existed in differentfrequency bandwidths within the FDD frame so that the time of DL HARQprocess to be taken may be constant to be eight subframes.

Meanwhile, in the TDD, the k and j may be different for each UL subframerespectively so that the time required for performing DL HARQ processmay not be constant.

The following table represents values of k+j in each of UL subframeswithin the TDD frame according to the UL-DL configurations. When a valuek for the subframe n refers to k(n), and j(n) for a value j, thefollowing Table 13 represents the subframe n in a form of {k(n)}+j(n).k(n) may be any one among the plurality of values.

TABLE 13 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 Max 0 — —{6} + 4 — {4} + 6 — — {6} + 4 — {4} + 10 6 1 — — {7, 6} + {4} + 6 — — —{7, 6} + {4} + — 11 4 4 6 2 — — {8, 7, 4, — — — — {8, 7, 4, — — 12 6} +4 6} + 4 3 — — {7, 6, {6, 5} + {5, 4} + — — — — — 15 11} + 4 4 4 4 — —{12, 8, 7, {6, 5, 4, — — — — — — 16 11} + 4 7} + 4 5 — — {13, 12, — — —— — — — 17 9, 8, 7, 5, 4, 11, 6} + 4 6 — — {7} + 4 {7} + 6 {5} + 5 — —{7} + 4 {7} + — 14 7 FDD {4} + {4} + {4} + 4 {4} + 4 {4} + 4 {4} + {4} +{4} + 4 {4} + {4} + 8 4 4 4 4 4 4

In the Table 13, ‘Max’ represents the maximum value of the values of{k(n)}+j(n) that are obtained in each of the UL subframes according tothe UL-DL configurations. For example, in UL-DL configuration 4, themaximum value of the values of {k(n)}+j(n) is 16.

Meanwhile, the number of DL HARQ processes that are operated in the DLis associated the maximum value of the values of {k(n)}+j(n). That is,the number of DL HARQ processes in a cell may be determined to be thenumber of DL subfrrames that are included within a section correspondingto the maximum value of the values of {k(n)}+j(n).

FIG. 18 illustrates an example of determining the number of DL HARQprocesses in a FDD cell.

Referring to FIG. 18, k=4 and j=4, and the number of DL subframes thatare included in a section of the subframe including the number of k+j is8. Since each of the eight DL subframes may be used in different DL HARQprocesses, the maximum number of DL HARQ processes (refer to M_(HARQ))may be 8.

FIG. 19 illustrates an example of determining the number of DL HARQprocesses for each of UL-DL configurations 0 to 2 in a TDD cell.

For example, according to UL-DL configuration 0, k is 6 and j is 4 basedon a UL subframe 191. And the number of the DL subframes that areincluded in the section of k+j subframes is 4. Therefore, M_(HARQ) is 4(M_(HARQ)=4).

FIG. 20 illustrates an example of determining the number of DL HARQprocesses for each of UL-DL configurations 3 to 5 in a TDD cell.

For example, according to UL-DL configuration 3, k is 11 and j is 4based on a UL subframe 201. And the number of the DL subframes that areincluded in the section of k+j subframes is 9. Therefore, M_(HARQ) is 9(M_(HARQ)=9).

FIG. 21 illustrates an example of determining the number of DL HARQprocesses for UL-DL configuration 6 a TDD cell.

For example, according to UL-DL configuration 6, k is 7 and j is 7 basedon a UL subframe 211. And the number of the DL subframes that areincluded in the section of k+j subframes is 6. Therefore, M_(HARQ) is 6(M_(HARQ)=6).

That is, FIGS. 18 to 21 represent examples of determining the number ofDL HARQ processes for a cell.

Meanwhile, in case of applying method 1 or 2 described above, theACK/NACK timing of a secondary cell may be different. For example, ifmethod 1 is applied for the case that a primary cell is a FDD cell and asecondary cell is a TDD cell, the ACK/NACK for data received in the DLsubframe of the secondary cell is transmitted to the UL subframe of theprimary cell, and at the moment, the UL subframe depends on the ACK/NACKtiming of the FDD cell that is a primary cell. Therefore, the bothtimings of the ACK/NACK timing in case that the TDD cell is exclusivelyconfigured and the ACK/NACK timing in case that cells that use differentframe structures are aggregated may be different with each other. Inthis case, it may be problem how to determine the number of DL HARQprocesses.

First of all, some technical terms are defined for the convenience ofdescriptions. Here, k_(s-p) represents a time difference between the DLsubframe of the secondary cell that receives data and the UL subframe ofthe primary cell that transmits the ACK/NACK for the data. And, j_(P-S)represents a time difference between the UL subframe of the primary cellthat transmits the ACK/NACK and the shortest DL subframe of thesecondary cell that is available to be transmitted by an identical DLHARQ process.

If a primary cell is the FDD cell and a secondary cell is the TDD celland the method 1 is used, j_(P-S) may be represented as follows for eachsubframe of the secondary cell.

TABLE 14 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 0 6/5 6 6/56 1 6/4 6 6/5 6/4 6 6/5 2 5/4 4 5 5/4 5/4 4 5 5/4 3 6/5 6/4 6/4 6/4 6/46/4 6 4 6/4 6/4 6/4 6/4 6/4 6/4 6 6/5 5 5/4 5/4 5/4 5/4 5/4 5/4 5 5/4 66/5 6 6/5 6/4 6

In the table, each subframe is represented as a form of A/B, here, Arefers to a value for the case that a collision of DL HARQ processes isnot allowed, and B refers to a value for the case that a collision of DLHARQ processes is allowed.

The following table represents k_(S-P)+j_(P-S) in the case that acollision of DL HARQ processes is not allowed for each subframe of thesecondary cell. That is, if k_(S-P) for subframe n is k_(S-P)(n) andj_(P-S) for subframe n is j_(P-S) (n), the following table refers tok_(S-P)(n)+j_(P-S)(n) for subframe n.

TABLE 15 UL-DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 Max DL MinDL 0 4 + 6 — — — 4 + 6 4 + 6 — — — 4 + 6 4 4 1 4 + 6 — — 4 + 6 4 + 6 4 +6 — — 4 + 6 4 + 6 6 6 2 4 + 5 — 4 + 5 4 + 5 4 + 5 4 + 5 — 4 + 5 4 + 54 + 5 8 7 3 4 + 6 4 + 6 4 + 6 4 + 6 4 + 6 4 + 6 — — — 4 + 6 7 7 4 4 + 64 + 6 4 + 6 4 + 6 4 + 6 4 + 6 — — 4 + 6 4 + 6 8 8 5 4 + 5 4 + 5 4 + 54 + 5 4 + 5 4 + 5 — 4 + 5 4 + 5 4 + 5 9 8 6 4 + 6 — 4 + 6 4 + 6 4 + 6 —4 + 6 5 5

In the above table, ‘Max DL’ represents the maximum value of the numberof DL subframes that are included in the section ofk_(S-P)(n)+j_(P-S)(n), which is fixed by each subframe in thecorresponding UL-DL configurations, and ‘Min DL’ represents the minimumvalue of the number of DL subframes that are included in the section ofk_(S-P)(n)+j_(P-S)(n), which is fixed by each subframe in thecorresponding UL-DL configurations.

In this case, the maximum number of DL HARQ processes that is applied tothe secondary cell corresponds to one of the followings.

i) With the maximum value of the number of DL subframes available insections corresponding to each k_(S-P)+j_(P-S), or the maximum value ofthe number of DL subframes included in sections corresponding to eachk_(S-P)+j_(P-S), the maximum number of DL HARQ processes that is appliedto the secondary cell may be determined. A valid DL subframe means a DLsubframe that is available to transmit data by a base station. That is,if a base station is unable to transmit data, it is not a valid DLsubframe even though it was set to be a DL subframe in the UL-DLconfigurations. In order for a specific DL subframe to be a valid DLsubframe, data channel transmission should be available in the specificDL subframe and also, the control channel transmission should beavailable in the DL subframe that is defined to transmit the controlchannel that schedules the data channel.

ii) Or with the minimum value of the number of DL subframes available insections corresponding to each k_(S-P)+j_(P-S), or the minimum value ofthe number of DL subframes included in sections corresponding to eachk_(S-P)+j_(P-S), the maximum number of DL HARQ processes that is appliedto the secondary cell may be determined.

For example, suppose a primary cell is the FDD cell and a secondary cellis the TDD cell based on UL-DL configuration 2. In this case, referringto Table 15, the value of k_(S-P)+j_(P-S) is 9 for each subframe of thesecondary cells, and the maximum value of the number of all DL subframesincluded in sections corresponding to k_(S-P)+j_(P-S) respectively is 8,and the minimum value is 7. At the moment, the maximum number of DL HARQprocesses of the secondary cell is set to be 8 or 7.

Such a method has a disadvantage that the DL HARQ process cannot beallocated in a part of DL subframes of the secondary cell, but there isan advantage in that data retransmission time can be shortened.

Or, the maximum number of DL HARQ processes of the secondary cell may bedetermined as follows. It is based on the premise that a primary cell (afirst serving cell) is the FDD cell and a second cell (a second servingcell) is the TDD cell.

FIG. 22 illustrates an example of determining the number of DL HARQprocesses of the secondary cell.

The UE receives data in a DL subframe of the secondary cell (the secondserving cell), and transmits an ACK/NACK signal in response to the datain a UL subframe of the primary cell (the first serving cell). At themoment, a first type of frame (a FDD frame) is used in the primary cell,and a second type of frame (a TDD frame) is used in the secondary cell.

In this case, the number of DL HARQ processes of the secondary cell iscalculated by the number of DL subframes that are included in each ofsections including a specific number of subframes based on each subframeincluded in the TDD frame of the secondary cell (step, S171), and basedon the number of the DL subframes, the number of DL HARQ processes ofthe secondary cell may be determined. Particularly, one of the followingmethods 1) to 4) can be applied.

1) In the FDD cell, the minimum time between the initial transmissionand retransmission by the DL HARQ is eight subframes. Considering this,in the secondary cell, the maximum number of DL HARQ processes of thesecondary cell may be determined by the maximum value of the number ofDL subframes that are included in the section of eight subframes.

2) Or, the maximum number f DL HARQ processes of the secondary cell maybe determined with the minimum value of the number of DL subframes thatare included in the section of eight subframes in the secondary cell.

The following table represents the number of DL subframes that areincluded in the section of eight subframes for each subframe accordingto the UL-DL configurations when the secondary cell is the TDD cell.

TABLE 16 UL-DL Subframe n # of DL subframes within 8 subframesConfiguration 0 1 2 3 4 5 6 7 8 9 0~ 1~ 2~ 3~ 4~ 5~ 6~ 7~ 8~ 9~ Max Min0 D D U U U D D U U U 4 3 2 3 4 4 3 2 3 4 4 2 1 D D U U D D D U U D 5 44 5 6 5 4 4 5 6 6 4 2 D D U D D D D U D D 6 6 6 7 7 6 6 6 7 7 7 6 3 D DU U U D D D D D 5 5 5 6 7 7 6 5 5 5 7 5 4 D D U U D D D D D D 6 6 6 7 87 6 6 6 6 8 6 5 D D U D D D D D D D 7 7 7 8 8 7 7 7 7 7 8 7 6 D D U U UD D U U D 4 3 3 4 5 5 4 3 4 5 5 3

Referring to the table 16, ‘# of DL subframes within 8 subframes’represents the number of DL subframes included in the section of thecorresponding 8 subframes depending on from which subframe the sectionof 8 subframes is started 0 to 9.

For example, if the section of 8 subframes is started from 0 based onUL-DL configuration 0 in Table 16, the number of DL subframes that areincluded in the section of 8 subframes is 4, and if the section of 8subframes is started from 1, the number of DL subframes that areincluded in the section of the corresponding 8 subframes is 3. And,‘Max’ represents the maximum value among the numbers of DL subframesthat are included in each section of 8 subframes for the correspondingUL-DL configuration, and ‘Min’ represents the minimum value among thenumbers of DL subframes that are included in each section of 8 subframesfor the corresponding UL-DL configuration.

3) Or the maximum number of DL HARQ processes of the secondary cell maybe determined based on an average value of the numbers of DL subframesthat are included in each section of 8 subframes in the secondary cell.For example, in Table 16, UL-DL configuration 0 is applied for thesecondary cell, the numbers of DL subframes that are included in eachsection of 8 subframes will be {4, 3, 2, 3, 4, 4, 3, 2, 3 and 4}. Byobtaining an average value of these values, the nearest integer numberof the average values may be determined to be the maximum number of DLHARQ processes of the secondary cell.

4) Or, a specific value between the maximum value and the minimum valueof the numbers of DL subframes that are included in each section of 8subframes in the secondary cell may be determined to be the maximumnumber of DL HARQ processes of the secondary cell. The specific valuemay be a predefined fixed value or a value signaled.

The more number of DL HARQ processes are there, the more utilization isavailable for the DL subframes that are included in the secondary cell.However, the soft buffer that stores data should be divided as many aspossible. Therefore, there is a disadvantage in that it requires a bigsoft buffer. Considering such a tradeoff, a method may be chosen to beused from the methods 1) to 4) described above.

Or the maximum number of DL HARQ processes of the secondary cell may bedetermined to be the number of DL subframes that are included in thesection of 10 subframes. Since an asynchronous HARQ is operated in DL, abase station freely set up the HARQ timing unlike a synchronous HARQ.For the sake of implementation, the maximum number of DL HARQ processesmay be determined by the number of DL subframes that are included in thesection of 10 subframes.

The following table represents the number of DL subframes that areincluded in the section of 10 subframes for the UL-DL configurationsthat are applied to the secondary cell.

TABLE 17 # of DL UL-DL subframes Config- Subframe n within uration 0 1 23 4 5 6 7 8 9 10 subframes 0 D D U U U D D U U U 4 1 D D U U D D D U U D6 2 D D U D D D D U D D 8 3 D D U U U D D D D D 7 4 D D U U D D D D D D8 5 D D U D D D D D D D 9 6 D D U U U D D U U D 5

Or, the maximum number of the DL HARQ processes of the secondary cellmay also be fixed to be eight, which is the same as that of the FDDcell. Particularly, this method may be applied in case that the FDD cellfollows the ACK/NACK timing which is applied to the FDD cell when theFDD cell transmits the ACK/NACK for the data received in the DL subframeof the secondary cell, which is the TDD cell. This method has anadvantage that the implementation is simple.

Or, the maximum number of the DL HARQ processes of the secondary cellmay be determined to be the same value as the maximum number of the DLHARQ processes applied when the secondary cell, which is the TDD cell,unilaterally used. Or, throughout all UL-DL configurations in Table 16above, the maximum or the minimum number of the DL subframes may bedetermined to be the number of the maximum DL HARQ processes of thesecondary cell.

Or, when the maximum value that divides the soft buffer is M_(limit),the M_(limit) value may also be determined to be the maximum number ofthe DL HARQ processes of the secondary cell. If the number of the DLHARQ processes of the secondary cell is set to be greater than themaximum division number, it cannot be actually operated because itexceeds the capacity.

Or, the maximum number of the DL HARQ processes of the secondary cell,which is the TDD cell, may be limited to eight or less. In the DCIformat that is scheduling the conventional FDD cell, 3 bits of HARQprocess number field may be existed, and in the DCI format that isscheduling the TDD cell, 4 bits of HARQ process number field may beexisted. If the maximum number of the DL HARQ processes of the secondarycell is limited to eight or less, 3 bits of HARQ process number field isincluded in the DCI format that is scheduling the TDD cell so as to bethe same size of the DCI format that is scheduling the FDD cell.

In the methods described above, it has been described under the premisethat a special subframe is applied as the DL subframe. However, this isnot intended to impose a limitation, but the special subframe may beexcluded when calculating the number of DL subframes included within apredefined subframe section. For example, the subframe in which DwPTSincluded in the special subframe is short as a predetermined value orless may be excluded.

In addition, in the methods described above, different methods may becombined and used for each UL-DL configuration. In addition, differentmethods may be applied according to whether the ACK/NACK timing is basedon method 1 or method 2.

FIG. 23 is a block diagram of a wireless apparatus in which theembodiments of the present invention is implemented.

A base station 100 includes a processor 110, a memory 120 and a radiofrequency (RF) unit 130. The processor 110 implements the proposedfunctions, processed, and/or methods. For example, the processor 110setup a plurality of serving cells that use different frame structurewith each other to a UE, and transmits data that requires ACK/NACKresponse through each of the serving cells. In addition, the processor110 determines the number of the DL HARQ processes for the secondarycell. The memory 120 is connected to the processor 110 and configured tostore various information used for the operations for the processor 110.The RF unit 130 is connected to the processor 110 and configured totransmit and/or receive a radio signal.

A UE 200 includes a processor 210, a memory 220, and a RF unit 230. Theprocessor 210 implements the proposed functions, processed, and/ormethods. For example, the processor 210 may receive the configuration ofa first serving cell and a second serving cell that use different framestructures with each other, receive data in the DL subframe of thesecond serving cell, and transmit the ACK/NACK signal in response to thedata in the UL subframe of the first serving cell. The number of DL HARQprocesses of the secondary cell is determined based on a value which issignaled before or the number of DL subframe included in the subframesection of a predetermined number in the secondary cell.

The processor 110, 210 may include Application-Specific IntegratedCircuits (ASICs), other chipsets, logic circuits, data processingdevices and/or converters for mutually converting baseband signals andradio signals. The memory 120, 220 may include Read-Only Memory (ROM),Random Access Memory (RAM), flash memory, memory cards, storage mediaand/or other storage devices. The RF unit 130, 230 may include one ormore antennas for transmitting and/or receiving radio signals. When anembodiment is implemented in software, the above-described scheme may beimplemented as a module (process, function, etc.) for performing theabove-described function. The module may be stored in the memory 120,220 and executed by the processor 110, 210. The memory 120, 220 may beplaced inside or outside the processor 110, 210 and connected to theprocessor 110, 210 using a variety of well-known means.

What is claimed is:
 1. A method for determining a number of hybridautomatic repeat request (HARQ) processes in a carrier aggregationsystem in which a plurality of serving cells are configured, the methodperformed by a user equipment and comprising: receiving data in adownlink subframe of a second serving cell; and transmitting a positiveacknowledgement/negative-acknowledgement (ACK/NACK) signal in responseto the data in an uplink subframe of a first serving cell, wherein thefirst serving cell uses a frequency division duplex (FDD) frame and thesecond serving cell uses either a time division duplex (TDD) frame or aFDD frame, wherein when the second serving cell uses the TDD frame, theTDD frame is configured with any one of uplink-downlink configurationsaccording to a table as follows: Uplink-downlink Subframe numberconfiguration 0 1 2 3 4 5 6 7 8 9 0 D S U U U D S U U U 1 D S U U D D SU U D 2 D S U D D D S U D D 3 D S U U U D D D D D 4 D S U U D D D D D D5 D S U D D D D D D D 6 D S U U U D S U U D

wherein D denotes a downlink subframe, S denotes a special subframe andU denotes an uplink subframe, and, wherein a maximum number of HARQprocesses of the second serving cell is determined as a maximum numberof HARQ processes of the first serving cell regardless of whether thesecond serving cell uses the TDD frame or the FDD frame.
 2. The methodof claim 1, wherein the maximum number of HARQ processes of the firstserving cell is
 8. 3. The method of claim 1, wherein the first servingcell is a primary cell that the user equipment performs an initialconnection establishment procedure or a connection reestablishmentprocedure with a base station, and wherein the second serving cell is asecondary cell which is additionally allocated to the user equipment inaddition to the primary cell.
 4. A user equipment, comprising: a radiofrequency (RF) unit configured to transmit and receive a radio signal;and a processor connected to the RF unit, wherein the processor isconfigured to: receive data in a downlink subframe of a second servingcell and transmit a positive acknowledgement/negative-acknowledgement(ACK/NACK) signal in response to the data in an uplink subframe of afirst serving cell, wherein the first serving cell uses a frequencydivision duplex (FDD) frame and the second serving cell uses either atime division duplex (TDD) frame or a FDD frame, wherein when the secondserving cell uses the TDD frame, the TDD frame is configured with anyone of uplink-downlink configurations according to a table as follows:Uplink-downlink Subframe number configuration 0 1 2 3 4 5 6 7 8 9 0 D SU U U D S U U U 1 D S U U D D S U U D 2 D S U D D D S U D D 3 D S U U UD D D D D 4 D S U U D D D D D D 5 D S U D D D D D D D 6 D S U U U D S UU D

wherein D denotes a downlink subframe, S denotes a special subframe andU denotes an uplink subframe, and, wherein a maximum number of HARQprocesses of the second serving cell is determined as a maximum numberof HARQ processes of the first serving cell regardless of whether thesecond serving cell uses the TDD frame or the FDD frame.
 5. The userequipment of claim 4, wherein the maximum number of HARQ processes ofthe first serving cell is
 8. 6. The user equipment of claim 4, whereinthe first serving cell is a primary cell that the user equipmentperforms an initial connection establishment procedure or a connectionreestablishment procedure with a base station, and wherein the secondserving cell is a secondary cell which is additionally allocated to theuser equipment in addition to the primary cell.
 7. A method forperforming hybrid automatic repeat request (HARQ) processes in a carrieraggregation system, the method performed by a base station andcomprising: transmitting data in a downlink subframe of a second servingcell; and receiving a positive acknowledgement/negative-acknowledgement(ACK/NACK) signal in response to the data in an uplink subframe of afirst serving cell, wherein when the second serving cell uses the TDDframe, the TDD frame is configured with any one of uplink-downlinkconfigurations according to a table as follows: Uplink-downlink Subframenumber configuration 0 1 2 3 4 5 6 7 8 9 0 D S U U U D S U U U 1 D S U UD D S U U D 2 D S U D D D S U D D 3 D S U U U D D D D D 4 D S U U D D DD D D 5 D S U D D D D D D D 6 D S U U U D S U U D

wherein D denotes a downlink subframe, S denotes a special subframe andU denotes an uplink subframe, and, wherein when the first serving celluses a frequency division duplex FDD frame, a maximum number of HARQprocesses of the second serving cell is determined as a maximum numberof HARQ processes of the first serving cell regardless of whether thesecond serving cell uses the TDD frame or the FDD frame.
 8. The methodof claim 7, wherein the maximum number of HARQ processes of the firstserving cell is
 8. 9. The method of claim 7, wherein the first servingcell is a primary cell that a user equipment performs an initialconnection establishment procedure or a connection reestablishmentprocedure with the base station, and wherein the second serving cell isa secondary cell which is additionally allocated to the user equipmentin addition to the primary cell.
 10. A base station, comprising: a radiofrequency (RF) unit configured to transmit and receive a radio signal;and a processor connected to the RF unit, wherein the processor isconfigured to: transmit data in a downlink subframe of a second servingcell and receive a positive acknowledgement/negative-acknowledgement(ACK/NACK) signal in response to the data in an uplink subframe of afirst serving cell, wherein when the second serving cell uses the TDDframe, the TDD frame is configured with any one of uplink-downlinkconfigurations according to a table as follows: Uplink-downlink Subframenumber configuration 0 1 2 3 4 5 6 7 8 9 0 D S U U U D S U U U 1 D S U UD D S U U D 2 D S U D D D S U D D 3 D S U U U D D D D D 4 D S U U D D DD D D 5 D S U D D D D D D D 6 D S U U U D S U U D

wherein D denotes a downlink subframe, S denotes a special subframe andU denotes an uplink subframe, and, wherein when the first serving celluses a frequency division duplex FDD frame, a maximum number of HARQprocesses of the second serving cell is determined as a maximum numberof HARQ processes of the first serving cell regardless of whether thesecond serving cell uses the TDD frame or the FDD frame.
 11. The basestation of claim 10, wherein the maximum number of HARQ processes of thefirst serving cell is
 8. 12. The base station of claim 10, wherein thefirst serving cell is a primary cell that a user equipment performs aninitial connection establishment procedure or a connectionreestablishment procedure with the base station, and wherein the secondserving cell is a secondary cell which is additionally allocated to theuser equipment in addition to the primary cell.